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Microwave sensors for scanning of sawn timber
Luleå tekniska universitet, Luleå.
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A microwave imaging sensor that measures the signal transmitted through a board was investigated with respect to the ability to predict the distribution of moisture and density in sawn lumber. Furthermore, the response from the sensor was related to strength properties of the boards. Multivariate statistics was used to relate the measured variables to various properties. A finite element model based on X-ray computed tomography images was developed to describe the interactions between microwaves and wood. The model made it possible to simulate the response from the sensor under varying conditions.

The results show that microwaves can be used for prediction of density and moisture content. They can also be used for prediction of strength properties, mainly from the correlation to density, but also from the influence on microwaves of structural variations in the wood. The finite element model is useful in the evaluation of microwave sensors for wood, drying equipment or other applications where electromagnetic waves interact with wood.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet , 2007. , 40 p.
Series
Doctoral thesis / Luleå University of Technology, 2007:9
Keyword [en]
microwave scanning; wood; density; moisture content; strength grading; finite element
National Category
Control Engineering
Identifiers
URN: urn:nbn:se:umu:diva-46102ISBN: 1402-1544 OAI: oai:DiVA.org:umu-46102DiVA: diva2:437018
Public defence
2007-04-18, Hörsal A, Forskargatan 1, Skellefteå, 10:00 (English)
Opponent
Supervisors
Available from: 2011-08-29 Created: 2011-08-26 Last updated: 2011-08-29Bibliographically approved
List of papers
1. Microwave penetration in wood using imaging sensor
Open this publication in new window or tab >>Microwave penetration in wood using imaging sensor
2005 (English)In: Measurement, Vol. 38, 15-20 p.Article in journal (Refereed) Published
Abstract [en]

It is possible to determine properties of wood using microwave scanning techniques. The purpose of this study was to verify the measured values from a microwave imaging sensor. Attenuation and phase shift of an electromagnetic wave transmitted through birch wood were measured and compared with theoretical calculated values. A test piece with varying thickness was measured with a scanner based on a microwave sensor (Satimo 9.375 GHz) at different temperatures and moisture contents. The density distribution of the test piece was determined by computer tomography scanning. The result showed good correspondence between measured and theoretical values. The proportion of noise was higher at low moisture content due to lower attenuation. There is more noise in attenuation measurement than in measurement of phase shift. A reason for this could be that wood is an inhomogeneous material in which reflections and scattering affect attenuation more than phase shift. The microwave scanner has to be calibrated to a known dielectric to quantify the error in the measurement. (C) 2005 Elsevier Ltd. All rights reserved.

Identifiers
urn:nbn:se:umu:diva-46077 (URN)
Available from: 2011-08-25 Created: 2011-08-25 Last updated: 2011-09-07
2. Finite element modeling (FEM) simulation of interactions between wood and microwaves
Open this publication in new window or tab >>Finite element modeling (FEM) simulation of interactions between wood and microwaves
2006 (English)In: Journal of Wood Science, Vol. 52, 406-410 p.Article in journal (Refereed) Published
Abstract [en]

The aim of this study was to use finite element modeling (FEM) as a tool to analyze microwave scattering in wood and to verify the model by measurements with a microwave scanner. A medical computed tomography scanner was used to measure distribution of density and moisture content in a piece of Scots pine (Pinus sylvestris). Dielectric properties were calculated from measured values for cross sections from the piece and used in the model. Images describing the distribution of the electric field and phase shift were obtained from the FEM simulation. The model was verified by measurements with a scanner based on a microwave sensor. The results show that simulated values correspond well to measured values. Furthermore, discontinuities in the material caused scattering in both the measured and the simulated values. The greater the discontinuity in the material, the greater was the need for computational power in the simulation.

Identifiers
urn:nbn:se:umu:diva-46078 (URN)
Available from: 2011-08-25 Created: 2011-08-25 Last updated: 2011-09-07
3. FEM simulation of interactions between microwaves and wood during thawing
Open this publication in new window or tab >>FEM simulation of interactions between microwaves and wood during thawing
2006 (English)In: 2nd Conference on Mathematical Modeling of Wave Phenomena / [ed] Nilsson B, Fishman L, New York: American Institute of Physics , 2006, 260-267 p.Chapter in book (Other academic)
Abstract [en]

Dipole polarization of water molecules is an important factor when microwaves interact with moist wood. Hence there will be a considerable change in dielectric properties when the wood changes from frozen to nonfrozen condition. The aim of this study was to develop a model that simulates measurements with a microwave scanner based on a sensor working at 9.4 GHz. Two-dimensional finite element modelling (FEM) was implemented to analyze interactions between microwaves and green wood during thawing of frozen wood at room temperature. A medical computed tomography scanner was used to measure the internal structure of density in a piece of wood in green and dry condition. From these density images the distribution of dry weight moisture content was calculated for a cross section of the piece and used in the model. Images describing the distribution of the electric field and phase shift at different temperatures where obtained from the FEM simulation. The results show that simulated values correspond well to measured values. This confirms that the model presented in this study is a useful tool to describe the interaction between microwaves and wood during microwave scanning at varying conditions.

Place, publisher, year, edition, pages
New York: American Institute of Physics, 2006
Identifiers
urn:nbn:se:umu:diva-46082 (URN)
Available from: 2011-08-25 Created: 2011-08-25 Last updated: 2011-09-08
4. FEM Simulation of Heating Wood In an Industrial Microwave Applicator
Open this publication in new window or tab >>FEM Simulation of Heating Wood In an Industrial Microwave Applicator
2005 (English)In: 10th International Conference on Microwave and High Frequency Heating, 2005, 415-418 p.Conference paper, Published paper (Other academic)
Identifiers
urn:nbn:se:umu:diva-46079 (URN)
Available from: 2011-08-25 Created: 2011-08-25 Last updated: 2011-09-07
5. Predicting moisture content and density distribution of Scots pine by microwave scanning of sawn timber II: evaluation of models generated on a pixel level
Open this publication in new window or tab >>Predicting moisture content and density distribution of Scots pine by microwave scanning of sawn timber II: evaluation of models generated on a pixel level
2006 (English)In: Journal of Wood Science, Vol. 52, 39-43 p.Article in journal (Refereed) Published
Abstract [en]

The purpose of this study was to use images from a microwave sensor on a pixel level for simultaneous prediction of moisture content and density of wood. The microwave sensor functions as a line-scan camera with a pixel size of 8mm. Boards of Scots pine (Pinus sylvestris), 25 and 50mm thick, were scanned at three different moisture contents. Dry density and moisture content for each pixel were calculated from measurements with a computed tomography scanner. It was possible to create models for prediction of density on a pixel level. Models for prediction of moisture content had to be based on average values over homogeneous regions. Accuracy will be improved if it is possible to make a classification of knots, heartwood, sapwood, etc., and calibrate different models for different types of wood. The limitations of the sensor used are high noise in amplitude measurements and the restriction to one period for phase measurements.

Identifiers
urn:nbn:se:umu:diva-46081 (URN)
Available from: 2011-08-25 Created: 2011-08-25 Last updated: 2011-09-07
6. Predicting the strength of Norway spruce by microwave scanning: A comparison with other scanning techniques
Open this publication in new window or tab >>Predicting the strength of Norway spruce by microwave scanning: A comparison with other scanning techniques
2007 (English)In: Wood and Fiber Science, Vol. 39, 167-172 p.Article in journal (Refereed) Published
Abstract [en]

In this study, 90 boards of Norway spruce (Picea abies (L.) Karst.) sized 48 x 148 mm in cross-section, have been examined using different scanning and measurement techniques. All of the boards originated from a sawmill located in southern Finland. Planar X-ray scanning, microwave scanning, and grain-angle measurement have been performed. In addition, strength and elastic properties were assessed for each piece by four point bending. The purpose of the study was to relate the potential of microwave scanning compared to other, industrially available techniques and to explain the physiological background of the microwave responses. The results show that the microwave signal, after transmission through wood, contains information about the bending strength and the modulus of elasticity. The correlation to density is a key factor. Annual ring width was also found to be correlated both to microwave measurements and strength properties.

Identifiers
urn:nbn:se:umu:diva-46080 (URN)
Available from: 2011-08-25 Created: 2011-08-25 Last updated: 2011-09-07
7. An online microwave scanner for sawn wood
Open this publication in new window or tab >>An online microwave scanner for sawn wood
2007 (English)Report (Other academic)
Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet/LTU Skellefteå/Träteknologi, 2007
Series
Teknisk rapport / Luleå tekniska universitet
Keyword
wood microwave scanning moisture content density
Identifiers
urn:nbn:se:umu:diva-46083 (URN)
Available from: 2011-08-25 Created: 2011-08-25 Last updated: 2011-09-08

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Citation style
  • apa
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